Parasitic friction and material wear exist in all moving parts, causing about 20% in global energy loss annually. Machinery startup accounts for a major portion of this loss. This issue involves a boundary lubrication problem, where rubbing surfaces are inadequately covered by lubricating oils. Lubricating oil fluids rely on tribochemical reactions to establish metalorganic tribofilms that protect the contacting surfaces. The improved oil lubrication mechanism can ensure smooth operation, improving efficiency, and extending the mechanical component lifetime. In this thesis, we study tribochemical reactions resulting from various fuel and oil blends. The interactions among blended additives are given particular attention. Lubrication phenomena are simulated using a ball-on-disk linear reciprocation configuration in a standardized tribological test rig, Optimol SRV5. The tribofilm growth patterns are investigated by measuring friction and electrical contact resistance (ECR), followed by a detailed surface analysis. The proposed lubrication mechanisms are verified with experimental and numerical simulation results. Fuel lubrication studies are conducted by investigating a) lubricity loss upon the addition of multiple oxygenated compounds, b) accelerated material wear rates observed in dieselethanol fuel blends, and c) enhanced lubrication performances with carbon-based nanofluid fuels. Lubricity loss is found to correlate with: ● Extended induction periods for ECR rises, ● Reduced average electrical contact resistance values, and ● Inhibitions of protective frictional species formations (e.g., iron oxides and graphite). The developed tribochemical reaction model advances the design of friction and extremepressure modifiers using tribo-active nanomaterials. For instance, adding carbon-based nanomaterials to fuels enhances lubrication performance by serving as tribo-active materials to accelerate tribofilm formation and by replenishing damaged surfaces. In engine oil systems, we demonstrated that the lubrication performance could be enhanced by formulating TiO2 nanoparticles modified by gallic acid esters, and polyether-based co(ter)polymers. Based on the tribochemical reaction mechanisms found in this study, we propose more designs of functionalized nanomaterials for advanced lubricant applications in future work.

<p>The present thesis thoroughly treats a special friction reduction phenomenon that may appear in boundary lubricated tribological contacts, of the type encountered in numerous mechanical components made of steel. The phenomenon involves the formation of a special type of tribofilm that offers very low coefficients of friction. Typically the friction level becomes halved when the film is formed, compared to when it is not formed. Since boundary lubricated mechanical components are so common in all sorts of machinery, the technical and economical potential of this phenomenon is gigantic.</p><p>The tribofilm is produced on the steel surface, resulting in friction coefficient reduction from typically 0.08–0.1 to 0.04–0.06. The tribofilm is formed from the metal in the carbon coating and sulfur in the oil additive. The main film studied was WS₂, which is a well-known low-friction material. It includes easy shearing atomic planes, in the same fashion as the solid lubricants MoS₂ and graphite. Virtually no carbon is present in the tribofilm, despite carbon being the main constituent of both the coating and the additive. No films form on the Me-DLC coated part.</p><p>It was also found that WSi₂-particles could result in the formation of WS₂-containing tribofilms. It was concluded that they, just as the W-DLC film, were sufficiently weak to mill down to very small particles, and chemically reactive in the prevailing tribological conditions. However, WC particles were too stable, both mechanically and chemically, to result in any film formation.</p><p>The chemical driving forces for formation of the tribofilms were analyzed using EkviCalc, a commercial software for thermodynamical calculations based on minimization of Gibbs free energy for a system as a function of temperature and pressure. The simulations indeed confirmed that both WS₂ and MoS₂ should be expected to be stable compounds, coexisting with FeS, in the studied environment. As a spin-off result, the thermodynamical calculations indicated that coatings of the Cr-C type should impose very little tribochemical wear of the uncoated steel surface, and even reduce the forma-tion of FeS (the “traditional” tribofilm) on the steel surface in S-containing environments. </p><p>As a final spin-off, the thermodynamical calculations indicate that the Ti-C coating should be very resistant to tribochemical wear in the S-containing environment. </p>

The present thesis thoroughly treats a special friction reduction phenomenon that may appear in boundary lubricated tribological contacts, of the type encountered in numerous mechanical components made of steel. The phenomenon involves the formation of a special type of tribofilm that offers very low coefficients of friction. Typically the friction level becomes halved when the film is formed, compared to when it is not formed. Since boundary lubricated mechanical components are so common in all sorts of machinery, the technical and economical potential of this phenomenon is gigantic. The tribofilm is produced on the steel surface, resulting in friction coefficient reduction from typically 0.08–0.1 to 0.04–0.06. The tribofilm is formed from the metal in the carbon coating and sulfur in the oil additive. The main film studied was WS2, which is a well-known low-friction material. It includes easy shearing atomic planes, in the same fashion as the solid lubricants MoS2 and graphite. Virtually no carbon is present in the tribofilm, despite carbon being the main constituent of both the coating and the additive. No films form on the Me-DLC coated part. It was also found that WSi2-particles could result in the formation of WS2-containing tribofilms. It was concluded that they, just as the W-DLC film, were sufficiently weak to mill down to very small particles, and chemically reactive in the prevailing tribological conditions. However, WC particles were too stable, both mechanically and chemically, to result in any film formation. The chemical driving forces for formation of the tribofilms were analyzed using EkviCalc, a commercial software for thermodynamical calculations based on minimization of Gibbs free energy for a system as a function of temperature and pressure. The simulations indeed confirmed that both WS2 and MoS2 should be expected to be stable compounds, coexisting with FeS, in the studied environment. As a spin-off result, the thermodynamical calculations indicated that coatings of the Cr-C type should impose very little tribochemical wear of the uncoated steel surface, and even reduce the forma-tion of FeS (the “traditional” tribofilm) on the steel surface in S-containing environments. As a final spin-off, the thermodynamical calculations indicate that the Ti-C coating should be very resistant to tribochemical wear in the S-containing environment.

Within pressurised Metered Dose Inhalers (pMDIs) the contact between the valve components and elastomeric seals is of major significance, representing the main contributory factor to the overall system frictional characteristics. Therefore, the seal performance is extremely important and must be optimised to meet the contradictory requirements of preventing leakage and allowing smooth actuation. The environmentally driven trend to HFA formulations as opposed to CFC based ones has deteriorated this problem due to poor lubrication conditions and it has, consequently, increased the frictional losses during the pMDI actuation (hysteresis cycle). Research has been conducted into the key areas of the inhaler mechanism. As such, the contact pressure distribution and resulting reactions have been investigated, with emphasis on the correct treatment of the elastomer (seal) characteristics. The modelling of the device has been conducted within the environment of the multibody dynamics commercial software ADAMS, where a virtual prototype has been built using solid CAD geometries of the valve components. An equation was extrapolated to describe the relation between the characteristics of the ultra thin film contact conditions (sliding velocity, surface geometry, film thickness and reaction force) encountered within the inhaler valve and integrated into the virtual prototype allowing the calculation of friction within the conjuncture (due to viscous shear and adhesion). The latter allowed the analysis and optimisation of key device parameters, such as seal geometry, lubricant properties etc. It has been concluded that the dominant mechanism of friction is adhesion, while boundary lubrication is the prevailing lubrication regime due to the poor surface roughness to film thickness ratio. The multibody dynamics model represents a novel multi physics approach to study the behaviour of pMDIs, including rigid body inertial dynamics, general elasticity, surface interactions (such as adhesion), hydrodynamics and intermolecular surface interactions (such as Van der Waals forces). Good agreement has been obtained against experimental results at component and device level.

Lubricants additives are inseparable component of modern high-tech lubricants. Viscosity index improvers additives play uncoverable role in a branch of boundary lubrication. They are designed to ensure sufficient oil film thickness in low speeds of frictional surfaces motion. This thesis deal with effect of specific viscosity index improvers additives on boundary lubrication film formation in elliptical contact. The purpose of this work is also checking the effect of concentration changing these additives on lubricantion film thickness formation.

Excavators which used as earthmoving machinery are exposed to heavy loads and operate long hours repeatedly. The forces that are transmitted trough pin bearings are observed to cause failure due to wear. Therefore, durability of bearings is crucial for excavators. The aim of this study is to perform wear analysis of excavator bearings running in boundary lubrication regime. A hydraulically powered test rig, which simulates the operating conditions of excavator bearings is designed and constructed. Wear is obtained applying both force and relative motion between pin and bearing on the test rig. Several tests are performed in this test bench. Force and material type are varied as the factors influencing wear. Results are compared and the effects of the factors on wear are determined.

Le contrôle de la friction en régime limite s’effectue par le biais de lubrifiants qui, par l’adsorption de molécules sur les surfaces, réduisent les contacts directs entre aspérités. Dans ce contexte, l’objectif de cette thèse est de comprendre et de coupler les mécanismes, à la fois, d’adsorption et d’auto-organisation de différents acides gras sur des surfaces, et les mécanismes de friction interfaciale sous des conditions stationnaires et transitoires. L’effet de l’architecture moléculaire, modifiée par la présence et la conformation d’une insaturation dans la chaîne aliphatique des acides gras, a également été analysé. La caractérisation in-situ, à l’échelle moléculaire, a été réalisée avec le tribomètre moléculaire ATLAS développé au LTDS. Cet appareil permet des déplacements quasi-statiques et dynamiques, d’une sphère mise en regard d’un plan, suivant trois axes. Trois solutions d’acides gras, en faible concentration dans du dodécane, ont été analysées. Les déplacements et les forces, normaux et tangentiels, sont mesurés à l’aide de capteurs capacitifs d’une résolution respective de 0.015 nm et 10 nN. Des sollicitations dynamiques superposées permettent de caractériser simultanément la rhéologie de l’interface confinée en termes d’amortissement et de raideur, dans les deux directions. Les résultats montrent que les acides gras s’adsorbent par interaction physique sur les surfaces pour former des films visco-élastiques d’une épaisseur d’environ 15 Å sur chaque surface. Le taux de couverture et la cinétique d’adsorption de ces couches dépendent de l’architecture moléculaire des acides gras. Cette dernière gouverne également la friction interfaciale, qui a été qualifiée de supraglissement, et la rhéologie des monocouches auto-assemblées. Les différentes organisations de films conduisent ainsi à différentes évolutions de la friction en fonction de la vitesse de glissement et de la pression de contact. La réponse en friction transitoire et l’accommodation lors du glissement vers un nouvel état stationnaire ont de plus été décrites par des distances caractéristiques, de plusieurs nanomètres, reflétant le renouvellement statistique des spots de contact, et par des temps de relaxation, de l’ordre de la seconde, décrivant le réarrangement moléculaire au sein de l’interface. Enfin, une modélisation théorique de la friction limite a été proposée afin de comprendre l’origine moléculaire de la friction entre monocouches d’acides gras mettant ainsi en évidence le couplage fort de deux échelles spatiales et temporelles. / Friction in boundary lubrication can be controlled by the adsorption of molecules on surfaces that reduce direct contacts between asperities. In this context, the aim of this thesis is to understand and to couple the mechanisms of adsorption and self-organization of different fatty acids on surfaces, with the mechanisms of interfacial friction under steady-state regime and transient conditions. The effect of the molecular architecture, modified by the presence and conformation of one unsaturation in the aliphatic chain of fatty acids, was also analyzed. In-situ characterization, at the molecular level, was performed with the molecular tribometer ATLAS, developed at LTDS. This apparatus allows quasi-static and dynamic displacements, of a sphere in front of a plane in three directions. Three fatty acids solutions, in low concentration in dodecane, were analyzed. The displacements and the forces, normal and tangential, are measured using capacitive sensors with a resolution of 0.015 nm and 10 nN respectively. Dynamic superimposed measurements allow a simultaneous rheological characterization of the confined interface in terms of damping and stiffness in two directions. The results show that the fatty acids adsorb on the surfaces by weak interactions and form viscoelastic films with a thickness of about 15 Å on each surface. The coverage rate and adsorption kinetics of these layers depend on the molecular architecture of the fatty acids. This architecture also governs interfacial friction, which has been described as superlubricity, and the rheology of self-assembled monolayers. Various film organizations have thus led to different evolutions of the friction as a function of the sliding speed and the contact pressure. The transient friction response and its accommodation during slip to a new stationary state have also been described by characteristic distances of several nanometers, reflecting the statistic average renewal of the contact spots, and by relaxation times of the order of the second, describing the molecular rearrangement within the interface. Finally, theoretical modeling of boundary friction has been proposed to understand the molecular origin of the friction between monolayers of fatty acids and highlight the strong coupling between both spatial and temporal scales.

The high frequency reciprocating rig (HFRR) is typically used to determine the lubricity of diesel fuels and has been accepted as the universal test apparatus in most countries of the world. A study was undertaken to analyse results obtained with the HFRR instrument by identifying crucial factors contributing to friction and wear mechanisms during a lubricity test under given conditions that may lead to a better understanding of test results obtained. The transient temperature distribution on a semiinfinite disk due to a circular continuous oscillatory heat source was also modelled. The model was used to calculate the contact temperature of a contact similar to what can be expected in the HFRR apparatus. For lubricated tests a mixture of n-hexadecane (cetane) as base fluid and palmitic acid as lubricity additive was used. Experimental results, including results obtained by running unlubricated (dry) tests and results obtained from literature were all considered to formulate the hypothesis that a fully functional additive film will not survive the full duration of the lubricity test, which is 75 minutes (ISO 12156-1, 2003). Friction values obtained in the experiments conducted were much higher than what is expected for boundary films formed by long chain carboxylic acids. Comparable friction values were obtained in the first few seconds of the tests, but the friction values increased indicating the destruction of the boundary film. Metal oxide lubrication dominates for the remainder of the test. Information of the exact mechanism prevailing in the crucial first few seconds of the test should be obtained by increasing the data acquisition rate of the apparatus. It is believed that junction growth through adhesion is the dominant factor in the initial stages of the test. GCxGC/TOF-MS showed the formation of hexane and heptane, which is a strong indication of the catalytic decomposition of the C16-hydrocarbons in the sample. / Dissertation (MEng)--University of Pretoria, 2010. / Chemical Engineering / unrestricted

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24605号 / 工博第5111号 / 新制||工||1978(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 平山 朋子, 教授 松原 厚, 教授 小森 雅晴 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The main objective for the work described in this PhD thesiswas to formulate a friction model to characterize thefrictional behavior of paper. More specifically, the modelshould explain a phenomenon that is typical for paper grades,viz.: that the level of paper-to-paper friction is dependent onthe direction and the number of previous slides. The modelshould also explain the lubricating effect oflow-molecular-mass lipophilic compounds (LLC) that occur inpaper on paper-to-paper friction. Furthermore, the model shoulddescribe the types of forces that influence paper-to-paperfiction and explain the mechanism by which the LLC decreasepaper-to-paper friction. This thesis consists of a literature review of the basicconcepts of tribology and a summary of the main results andconclusions from four studies on the frictional characteristicsof paper and a study on the friction and adhesion of cellulosesurfaces together with some unpublished material. The purpose of the investigation described in the firstpaper was to explain a phenomenon that is typical for papergrades, viz.: the level of paper-to-paper friction is dependenton the direction and the number of previous slides. Thisbehavior is called“friction hysteresis”by theauthors, and it has its origin in the reorientation of thefibers on the surface of a paper and their alignment relativeto the sliding direction. The second paper describes a study that was aimed atidentifying lubricants that occur natively in paper. Filterpapers were impregnated with model compounds representing woodextractives, i.e. low-molecular-weight lipophilic compounds,which are present in wood, pulp and paper, and thepaper-to-paper friction was determined. The results of thatstudy show that a wood extractive must fulfill severalstructural criteria in order to lubricate a paper surface: Itmust have a hydrophilic group that can attach to the papersurface and a linear hydrocarbon backbone of sufficientlength. Although it is not specifically stated in the second paper,the authors proposed a type of lubrication by which woodextractives decrease paper-to-paper friction that is, ineffect, boundary lubrication. The purpose of the investigationdescribed in the third paper was to clarify whether woodextractives and other low-molecular-mass lipophilic compoundsthat occur in paper can act as boundary lubricants on papersurfaces. The main objective of that study was to investigatethe role of chemical structure of LLC for their orientationrelative to the paper surface, which is an important criterionfor boundary lubrication. Filter papers were impregnated withmagnesium salts of different lipophilic acids, which were usedfor model compounds for the LLC. The deposited layers ofmagnesium salts were characterized by X-ray photoelectronspectroscopy (XPS) and contact angle goniometry and thefriction of the impregnated paper sheets was determined. Theresults show that the degree of lubrication and the resistanceto wear of the layers of a magnesium salt increased withincreasing chain length and increasing degree of linearity ofthe lipophilic acid. Based on the results of that study and ofearlier studies, it is concluded that boundary lubrication isthe type of lubrication by which low-molecular-mass lipophiliccompounds that occur natively in paper decreasepaper-to-paper-friction. In boundary lubrication, surfaces are covered withmonolayers of lubricant molecules that comprise an active headgroup that can attach to the surface, e.g. a carboxyl group,and an inert linear backbone, such as a long saturatedhydrocarbon chain. Such compounds form ordered monolayers onsurfaces, so that the backbone points vertically out of planeof the surface. The friction is then determined by theinteractions between the monolayers, which are weaker than theinteractions between the clean surfaces and this gives a lowerfriction. The fourth paper describes a study on the origin of thedifferences in friction levels between different linerboardsbased on recycled fiber (old corrugated container, OCC). Thesheets were subjected to two extraction stages and analyzedwith respect to surface roughness and their content oflow-molecular-mass lipophilic compounds (LLC). The resultsshowed that a high amount of LLC in the sheets lead to lowfriction, due to lubrication. The fifth paper describes a study that was aimed atdetermining the types of forces that influence the frictionbetween the surfaces of hydrophilic polymers and explaining themechanism by which boundary lubricants decrease the friction.The adhesion and the friction of model systems was measuredwith atomic force microscopy (AFM) using regenerated cellulosefilms and functionalised AFM tips and the effect of fatty acidsand humidity was investigated. The friction significantlyincreased with increasing humidity and that there was a strongcorrelation between the ability of a fatty acid to form ahydrophobic surface and its lubricating performance. Measuredadhesion forces at high humidity were well predicted bytheoretical models that took into account the effect of theLaplace pressure acting in a water meniscus formed aroundcontact regions due to capillary condensation. The resultsindicated that the degree of capillary condensation may beeffectively suppressed by increasing the hydrophobicity of thecontacting surfaces, causing adhesion and friction to decrease.These results suggest that friction between paper surfacesunder ambient conditions is greatly influenced by the degree ofcapillary condensation. Furthermore, lubrication by fatty acidsis achieved by the formation of a vertically oriented,hydrophobic monolayer that can withstand the stresses duringsliding and increase the hydrophobicity of the paper surfaceand thereby suppress capillary condensation. <b>Keywords:</b>Friction, paper-to-paper friction, frictionhysteresis, fibers, orientation, sliding direction, woodextractives, low-molecular-mass lipophilic compounds, boundarylubrication, adhesion, capillary condensation, Laplacepressure, surface forces, JKR theory, gas chromatography-massspectroscopy, X-ray photoelectron spectroscopy, contact angle,atomic force microscopy